Using a turn table or "lazy susan" is one of the best tips I can give you. They have a swivel base so you can easily turn what ever you are painting to get to all sides. You can use a piece of cardboard, but they will bend and you can't pick the cardboard up easily to move the wet rock if you wanted to. I bough a bunch of these off a lady on eBay and then found them at Wall-Mart for a few dollars each. Now I have a bunch of them. I can have many projects going at once. Just pick the turntable up and move it out of the way to work on another one.
I LOVE THESE THINGS!
You'll need something to put the paint on. You can use plastic butter tubs, paper plates, plastic lids,or buy these inexpensive plastic containers. They come in all sizes. The large round one even has a see thru cover to help keep the paint from drying up so quickly. I only use these if I am going to use all the paint quickly, otherwise I use a wet pallet for my paint.
Besides the turn tables, this would be my second biggest tip to you. These wet pallets will keep your paints wet for weeks. If you use a regular pallet the paint will dry up quickly from the air. These have a wet sponge under the "special" paper to keep the paint moist. They also have a cover for when you are not using them. You can save a lot of paint by using these things. They come in various sizes. The small one, (the "Handy" size above) I use the most, as it doesn't take up a lot of room. This one cost $7.96 from Mister Art.com
This is a larger one. Normally it runs about $29, but I used a 40% off coupon to get it. See how much bigger it is than the one I normally use (on the table next to the box for comparison). Mister Art also carries this size, but I bought this long ago at a store in town.
This is the one I use everyday. The Handy size about 8 1/2" X 7" You can see it gets a lot of use! I just rinse the paper off after I change projects or colors. I can reuse the same paper at least 3 or 4 times before having to replace it. Paper refills are onl $3.20 for 40 sheets. The sponges last a long time and are $5.56 for a set of 3.